Are planes going to get significantly less noisy? Not any time soon.

Date added: July 24, 2013

In a BBC article, their technology reporter looks at work being done to try and make aircraft engines less noisy. He describes the sound deadening liner casings of engines – but which increase the mass and the drag of the aircraft and that increases fuel burn. Another approach is making the engines bigger, as a bigger, slower engine and fans makes a lower and less offensive noise – but that is squeezing the room left for the liners, which paradoxically need to be thicker to cope with the lower sound levels. “We have reached the point of no return without redesigning the airframes” said to Prof Jeremy Astley, director of Southampton University’s Institute for Sound and Vibration Research (ISVR). Unless they go back to open rotor engines. Airlines have some concern about the amount of noise their planes generate, because the noise level of an aircraft can be a major factor in securing commercially vital slots at major airports. “At the London airports noisier aircraft get fewer slots” according to Prof Astley. In reality now, without profound changes to engines and airframes, for the foreseeable future, only marginal reductions in noise will be obtained.

How are aeroplanes getting quieter?

By Zoe KleinmanTechnology reporter, BBC News

Standing in the half-light and eerie silence of a semi-anechoic chamber – designed to absorb sound – in the bowels of the University of Southampton, I’m beginning to wonder whether I’ve stumbled upon a real-life Stargate.

At closer inspection, the silver semicircle at the back of the room contains a microphone rig rather than a wormhole leading to other planets, and the instrument pointing at it is an enormously high-pressured air jet, not a teleporter.

Computers in the adjoining office can create 3D soundscapes of the noise recorded inside the chamber.

The £500,000 facility is one of a suite of testing labs built at the university’s Institute of Sound and Vibration Research (ISVR) in an effort to create quieter aircraft.

It was created when Rolls-Royce asked the team to explore a potential noise issue regarding one specific valve on a particular turbo fan engine, explains Prof Jeremy Astley, director of the ISVR.

The valve’s job was to blast extremely hot air out of the the engine.

“In previous engines there were other noise sources so we hadn’t noticed this one,” he said.

“In noise control when you bring one source down others start protruding. We weren’t sure how much noise this made so we designed the facility to mount it and test it.”

Their conclusion was that the valve did indeed generate “a lot” of noise.

“But we also found it was easy to reduce – with perforated sheets.”

The facility is now a permanent fixture for teaching and research purposes, the engine in question is quieter as a result – and for Prof Astley, it’s an example of the benefits of academics working more closely with industry.

“You have the sure knowledge that what you are doing is not a waste of time,” he said.

“If the work was not of interest, it would not be supported.”

Community care

While sound levels have fallen by about 30 decibels since the early commercial flights, modern jet engines still register up to 160 decibels – much louder than the rowdiest of rock concerts at 115 decibels – and are at their most audible from the ground during take-off and landing, as anyone who has ever lived near an airport will tell you.

In August last year, more than 1,800 complaints were received about the noise around Heathrow – an increase from just 400 the year before – but for the aviation industry, keeping quiet is not only a matter of maintaining community relations.

The noise level of an aircraft is also a major factor in securing commercially vital slots at major airports.

“At the London airports noisier aircraft get fewer slots,” said the professor.

“The loudest ones are the oldest – the 747s. It was the best technology we had 35 years ago.”

While fuel efficiency and carbon emission levels are also taken into account, he cites noise as the “single most significant environmental restraint” in the expansion of commercial air transport.

Stale mate

The problem is that traditional engine quietening solutions are approaching stalemate.

Liner casings have long used on board aircraft to absorb sound.

Basic acoustic absorbers known as liners – small, light, honeycombed lattices containing millions of tiny air spaces which absorb sound, have been commonly used by commercial aircraft since the 1970s.

“You can always make an engine quieter with more liners,” said Prof Astley.

“But then you’re increasing the mass and the drag [of the aircraft] and that increases fuel burn. You have to have a very balanced approach [to environmental factors].”

Another tried and tested approach to noise control – making the engines bigger, as a bigger, slower engine and fans makes a lower and less offensive noise – is squeezing the room left for the liners, which paradoxically need to be thicker to cope with the lower sound levels.

“We have reached the point of no return without redesigning the airframes.”

Back to the future

The answer could lie in the revival of a radical idea first mooted more than 30 years ago.

Open rotor engines, containing contra-rotating propellers, are, as their name suggests, not enclosed.

Ironically the idea was originally shelved in the 1980s because the engines were thought to be too noisy – but Prof Astley dismisses the first prototypes as “primitive” by today’s standards.

In closed engines, one factor which can minimise noise is the flow of air around the engine rather than through it – known as the bypass ratio.

With an open engine, that ratio would become “infinite”, according to the professor.

Another bonus is that these engines could be up to 30% more fuel efficient than current models – although an aircraft redesign would still be inevitable.

“It would change both the appearance and the sound,” said Prof Astley.

“The sound would be more tonal – it would change not just the level but also the quality of noise.”

But it’s not likely to happen overnight.

“We are 90% of the way there. Perhaps we’ll start to see them on the next generation of single aisle aircraft – in about 10 years’ time.”

Researchers in South Korea are working on an idea once thought unlikely: a window which lets air pass through it while dramatically reducing the level of noise.

Sang-Hoon Kima, from Mokpo National Maritime University, and Seong-Hyun Lee, from the Korea Institute of Machinery and Materials, designed an acrylic resonance chamber, a bit like double glazing, which weakens sound at certain frequencies.

The window can be “tuned” to lessen lower frequencies by drilling small holes in the acrylic.

The researchers say their design is simple enough that an ordinary carpenter can make it, and suggest the concept could even be used to allow owners of homes by the sea to hear the sound of the waves, but to screen out other less attractive sounds from outside.

It is obvious that today’s aircraft manufacturers have excellent opportunities to market new product designs; escalating fuel costs, rising demand and the need for more quiet and environmentally friendly aircraft. A quick scan of scientific literature, press releases and the internet, reveals numerous of ideas for new aircraft and associated technology. Obviously, many of these are promising, especially from an environmentalists point of view because these aircraft are expected to deliver significant reductions in fuel consumption and noise emission. Question is whether any of these ideas can be taken seriously or could even be turned into a realistic product or operational prototype.

If we take a look into the future, it should be said that many of the technologies that can help make aircraft for example more noise sensitive, are already with us today. Others will have to be developed and introduced on the short term if they are to be delivering benefits on the short- to- mid-term.

As is seen in the above mentioned figure, a significant reduction in aircraft noise has been achieved over the last 50 years. However, the thing that catches the attention is the gradual flattening of the noise improvement curve.

In other words, the rate at which noise can be reduced at source is slowing down and it even seems that by now only marginal benefits are obtained.

Does this mean that our technological ability to reduce noise further is running into design limitations? Ever since the 1970s, the current, second generation of aircraft have been continuously developed and almost every aspect of the design that possessed some sort of improvement potential have been taken on to improve the overall design of the aircraft.

By now the further development of the current generation of aircraft seems to be of only marginal order and requires the application of new materials, techniques and designs.

Let us first consider how further, significant, improvements can be achieved. Considering what has been stated previously, this will require the introduction of aircraft that entirely deviate from the aircraft design paradigm of a fuselage with two wings, a tail, and powered by conventional turbofan engines.

Instead, manufacturers are now looking at designs that would change the face of the industry, featuring aircraft that can carry many passengers against a fraction of the amount of fuel burned and noise produced compared to current aircraft in operation.

In fact, there are already several aircraft manufacturers working on aircraft designs that entirely deviate from this design paradigm. Boeing Phantom Works has already started flying a scaled model of their Blended Wing Body (BWB) while Airbus has experimented with a redesigned Airbus A300 flying on hydrogen. Especially Boeings ‘Flying Wing’ concept receives much attention from the industry as this design is expected to deliver the greatest advantages.

However, the availability of such aircraft will take a substantial amount of time. Safety is one of the prime concerns when it comes to the operation of aircraft and cannot be compromised in any way. New designs that feature many new (not yet proven) technologies will have to go through a process which encompasses designing and building the aircraft, type certification and entry into service. Although this process is similar to that of already proven design this will take longer to complete than ‘regular’ designs. It is a fact that the time between a design on the drawing board and the actual delivery of the first production type might take up to 20 to 30 years, much longer than designs that are based on existing models.

It is therefore obvious that radical new aircraft designs such as the Blended Wing Body or Cryoplane will not appear on the commercial aviation market in the near future.

Although this might sound low spirited, especially with major technological developments continuing to take place, it is a realistic reflection of the general technological implementation rate that has been observed during the last decades. Furthermore, when aircraft such as those mentioned above are developed, it will take a significant amount of time before the benefits of these new, more efficient aircraft become evident compared to older generation aircraft, even far beyond the time these aircraft are introduced.

Instead, many aircraft manufacturers focus on developing aircraft that still have the design characteristics of today’s existing aircraft but feature new technological advances to gain significant improvements.

Interesting examples are the Airbus A350 and Boeing 787, that both have an appearance which is similar to the Airbus A330 and Boeing 767/777 but feature new advancements in its overall operating technology. Both development time and costs are substantially lower, the aircraft is available much sooner and thus delivers benefits in a smaller time-frame. Through the use of these new technologies, both manufacturers are able to achieve reductions on the order of 16dB below Chapter 4 requirements.

Aircraft of the future or an impossible dream

Recapitulating, it is obvious that the introduction of more technological advanced aircraft like for example the Blended Wing Body takes significantly more than 10 years to develop.

Furthermore, aircraft are valuable assets and are build to last long, with many aircraft staying in service for at least 25 to 30 years, and in some cases even longer.

That means that at the time new, more advanced aircraft enter the commercial aviation market, an additional 25 years will have passed before an old fleet of aircraft is fully replaced by this new technology.

In addition, we now talk solely about aircraft evolution, leaving developments in the airport environment aside. Looking at ultra-high capacity aircraft like the Blended Wing Body for example, requires much more than a small change in the overall aviation system.

The current system is tailored to the accommodation of aircraft not much larger than the A380, and tremendous obstructions are foreseen when even larger sized aircraft have to be handled through the system. The magnitude of these obstructions range from lack of airport capacity, increased ATC separation due to increased vortices generation, and airport lock-in effects.

How much quieter is the A380 at Heathrow, compared to the 747 Jumbo jets?

” the A380 promising to be far quieter than the Boeing 747.”

It depends where you are Its true inside the plane and on the ground when the plane takes off.

However, when landing, although the noise is reduced enough to get the A380 a good noise classification based on the ICAO tests 2 km from touchdown, for many of the neighbourhoods in West London that the plane flies over the A380 is as noisy or noisier than the old 747.

which is the ERCD REPORT 1106 : Noise Data for the First Three Years of Scheduled Airbus A380 Operations at London Heathrow Airport – by D P Rhodes

Noise restrictions should be based on what people hear and not on what tricks the airlines and ICAO come up with to convince them that new planes are quiet.

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The above report (page 4) says

“Figure 6 plots the A380 arrival noise data against the Boeing 747-400. As expected
the A380-800 Engine Alliance variant is 3-5dB quieter than the Boeing 747-400.
However, the Rolls powered A380 is around 2-3dB quieter, which is less than
expected.”

and its conclusion (page 6) says:

CONCLUSIONS
6.1 The report presents summary information on monitored noise levels for the first three
years of operation of the Airbus A380-800. Data has been compared to the Boeing
747-400, whose operations are most likely to be replaced by the A380 in the coming
years, and with other aircraft with a departure noise classification of QC/2.

6.2 The departure noise data confirms that the Airbus A380 is quieter than the Boeing
747-400, and although in some circumstances is noisier than other QC/2 aircraft due
to the longer distances currently flown, the noise performance is comparable with
other QC/2 aircraft on departure after accounting for distance flown. As a wider
number of operators introduce the aircraft in future years, it is expected that the
average distance flown will reduce, reducing average departure noise levels.

6.3 The approach noise data also confirms also confirms that the Airbus A380 is quieter
than the Boeing 747-400. However, there is a wider than expected difference
between the two engine types available on the A380. The Engine Alliance powered
A380 is also quieter than the Airbus A340-600 and Boeing 777-300ER, whereas the
Roll Royce powered variant is slightly noisier.

.

Data shows that some A380s are several decibels quieter than 747s some 9 -10 miles (Putney) from touchdown, but no less noisy further out.

4.5.2012The Dreamliner 787 has been much hyped, for its theoretical reduction in fuel use and in noise. Boeing claims it is 60% quieter, a statement that needs to be understood in terms of how aircraft noise is measured. It does not mean 60% quieter, in the way a layperson would understand the statement. It means actually a reduction of perhaps 3 decibels, to anyone standing under the flight path – a difference that is barely noticeable, even to trained ears. These figures are also theoretical, along the lines of the car fuel consumption figures given by manufacturers, and very difficult to replicate in real life. If the planes are heavier, taking off with more fuel and luggage on board, or landing on full power, they are still very noisy. And if there are more flights overhead, that is actually what people are bothered by, rather than a 3dB difference.http://www.airportwatch.org.uk/?p=1796